Vinyl siding was introduced in the early 1960's, but did not gain much attention until the 1970's. During the 1980's and 1990's, its use increased dramatically. Siding is the second largest market for polyvinyl chloride (“PVC”) resin, after pipe manufacturing, with approximately one billion pounds of resin used for this purpose in 1992. Vinyl siding is manufactured by a co-extrusion process in which two layers of PVC are generated in a continuous manner. The top layer called a capstock layer, comprises about one-quarter to one-third of the siding thickness and includes about 10% titanium dioxide, which is an opaque filler that provides some measure of UV protection for the lower layer, typically called the substrate. The substrate typically includes about 15% calcium carbonate, which balances the weight of the titanium dioxide to keep both extrusion streams equally fluid during manufacturing. A small quantity of a stabilizer such as tin mercaptan or butadiene is added as a stabilizer to chemically tie up any hydrochloric acid that is released into the PVC material as the siding ages. Lubricants may also be added to aid in the extrusion manufacturing process.
Vinyl siding is often considered a cheap substitute for wood, and is favored in some markets because of its lower maintenance requirement, it generally doesn't need to be painted or stained like real wood. Nevertheless, due to sun exposure, vinyl siding colors can dull and the surface of siding can become chalky.
Accordingly, artisans have continued to develop improved resin formulations for capstock layers to minimize the degradation of siding, decking, window, and fencing products made from PVC due to exposure of weather conditions including moisture and sunlight. For example, Solarkote® PB acrylic capstock resin from Atoglas of Philadelphia, Pa. has been commercialized as a capstock resin for imparting long lasting weatherability and enhancing the color and appearance of PVC substrates. When blended with PVC resin and co-extruded over PVC substrates as a capstock, Atoglas claims its Solarkote® PB acrylic resin upgrades PVC properties for exterior building products. The manufacturer also claims that Solarkote® PB/PVC blends impart weathering performance in medium dark colors where chalking can be an issue. Additionally, Atoglas' literature states that this coating exhibits improved color hold in whites—even at higher elevations in locations such as Arizona and Colorado. Solarkote® Acrylic capstock resin is advertised as screening out UV radiation in layers as thin as 6 mil (0.006 in). It is further suggested that this resin is suited for co-extruded applications for use in watercraft, recreational vehicles, lawn and garden products, building construction, automotive accessories, outdoor enclosures, tubs, spas and pool accessories.
Atoglas advertises that its Solarkote® capstock can be provided in a multi-layered form to often eliminate the need for painting. This three-layer co-extruded structure includes a polycarbonate, ABS, HIPS, acrylic-styrene-acrylonitrile, polycarbonate, PETG, or vinyl, plus two acrylic layers. Autoglas also advertises that by extruding the pigment in the center acrylic layer, the color effect has more depth and is better protected from scratches and weathering problems. The top layer is suggested as including a UV package that protects the pigment in the subsequent layer and the Solarkote® acrylic resins permit the designer to choose between matte, gloss and frosted effects. See trade literature articles entitled “Acrylic Resin Improves PVC Substrate Appearance” Thomas Net Industrial Newsroom Aug. 16, 2002 and “New Solarkote® PV Acrylic Capstock Resin from Autoglas Improves Weatherability and Appearance of PVC Substrates” Thomas Net Industrial Newsroom Aug. 7, 2002, (www.news.thomasnet.com) and trade literature entitled “Capstocks, Films Vie to Protect Outdoor Protects, Modern Plastics Worldwide Copyright 2003, (www.modplas.com/inc/mparticle).
In addition to color retention, manufacturers have also been focusing on producing vinyl building products with simulated wood-grain or other multi-colored appearances. Various methods of manufacturing such plastic articles have been used, but very few have succeeded in achieving a realistic wood-grain appearance in which the wood grain is strongly accented against a background layer. Typical processes involve blended colors including streaked, semi-transparent stained wood appearances often achieved through a multi-viscosity fusion process. This multi-viscosity fusion process uses multiple colorants of different viscosity, or thicknesses. Each color flows and mixes differently in the extruder. The first, usually a lighter color, melts easily and quickly and, thus, forms the background color for the siding. The second, usually a darker color, melts later in the extrusion process and streaks because it does not disperse completely. The blended effect is actually the darker color sitting on top of the lighter base color, creating a semi-transparent stained wood appearance. See, for example, U.S. Pat. No. 4,048,101 to Nakamachi, assigned to Daicel Ltd., and U.S. Pat. No. 5,387,381 to Saloom, assigned to Alcan Aluminum Corporation, and trade literature entitled Wolverine Master Craftsmen Education and Development Program© 2002 Wolverine, page 12 (www.siding.com).
Applicants have also experimented with using transparent pigment blends having different vicat softening temperatures based on poly-alpha-methylstyrene or other polymers, such as acrylic, acrylic-styrene acrylonitrile (ASA), acrylonitrile-ethylene-propylene-styrene (AES) or other transparent, opaque and/or translucent, weatherable polymers, or combinations of PVC and one or more of the above-noted polymers. See U.S. Pat. No. 6,752,941.
Through its experience in manufacturing PVC siding with ASA capstock, CertainTeed Corporation has observed that existing ASA capstock material has a fairly narrow processing window during extrusion with unfortunate consequences if the processing window is missed. If the processing temperature is too high during the manufacturing of such products, the melt viscosity of the capstock will become sticky; if the temperature is too low, the melt flow will center around the die.
There is also a noticeable difference in the coefficient of linear thermal expansion between ASA and PVC. This can lead to a difference in contraction rates during cooling between the predominantly ASA capstock and the predominantly PVC substrate, which can lead to undesirable bowing or dimensional distortion of the building material. It has also been noticed that for many variegated products using streaker pigment in acrylic based capstocks, it is inherently difficult to control the melt rheology in the ASA matrix for consistent streak wood grain patterns.
Notwithstanding the thermal contraction rate problem between ASA and PVD in siding products, others have developed techniques of providing wood grains by printing. See U.S. Pat. No. 6,823,794 to Bosler et al., and assigned to Bosler Designs, Inc. The '794 patent describes a method for printing an extruded sheet which includes the steps of extruding a sheet having an elevated temperature in the range of about 250° F. to about 450° F. and applying a heat curable ink or evaporable ink over the extruded sheet having the elevated temperature whereby the ink is cured immediately upon contact. The machinery selected by Bosler et al. can include multiple print rollers for printing in order to formulate a combination of colors. A plastisol ink which cures at a temperature of about 140° F. to about 200° F. along with evaporable inks and any conventional ink which cures upon contact with a material having an elevated temperature is suggested.
While printing variegated colors and textures such as wood grains could alleviate the above-mentioned consistency problem associated with providing streaked stained wood appearances through a multi-viscosity fusion process, there is no suggestion in Bosler et al. for overcoming the distortion problems caused by differences in the coefficient of liner thermal expansion. In fact, the '794 patent mentions that its extrudable plastic substrate material can include ASA, and there is no recognition in Bosler et al. regarding the contraction rate problem or its solution.
Accordingly, there is a present need for an improved capstock material which provides the weatherability of traditional co-extruded coatings and which enables detailed variegated patterns, without distortion or dimensional tolerance problems, or melt rheology complications.